(1) Field of the Invention
The present invention relates to a composite hollow valve for reciprocating engines such as an intake or exhaust valve. This invention also relates to a method for manufacturing the same.
(2) Description of the Prior Art
An internal combustion engine is required to have its intake and exhaust valves made lightweight to enhance responsibility and performance at low speed ranges. Various development have so far been proposed along this line. One such an example is disclosed in published Japanese patent application 61-229 907 by the same applicant as this invention, which teaches valve drive system parts including titanium aluminides intake and exhaust valves. This prior art relies upon the excellent properties of titanium-aluminides such as a low specific gravity equivalent to 48 to 61% of that of steel and oxidation resistance at high temperatures.
Production of a titanium aluminides engine valve such as an intake or exhaust valve consists in, first, providing a titanium aluminide billet as by argon arc melting, plasma arc melting, or vacuum arc melting method and then is formed into shape by precision casting, isothermal forging, extrusion, hot isostatic press, with, if desired, suitable thermal treatment and machining work depending on the type of the part being manufactured.
Different parts of an engine valve including a valve head, stem, stem end are required to have a different set of properties from one another for desired valve performance as a whole. For example, a valve head portion must be highly heat-resistant while increased mechanical strength is one of the requirements for valve stem portions. On the other hand, a valve stem end is required to have abrasive resistant properties.
Therefore, producing the whole parts of the engine valve from expensive titanium aluminides would not be said an advantageous manufacturing method in the sense of effective material use.
Furthermore, the forming of an engine valve in integrated whole by isothermal forging method presents a difficulty in the forging of a titanium aluminides, as an example, having an essential characteristics of a tough working, and, still furthermore, requires a skilled experience in a special forging technique even through the forming is available.
The present invention has been proposed to eliminate the above-mentioned drawbacks of the conventional engine valves.
It is therefore an objective of the present invention to provide a composite hollow engine valve for internal combustion engines such as an intake or exhaust valve which can be manufactured at a low cost while to a weight equal or substantially lighter than those prior art engine valves of comparable capacity and size.
It is also another object of the present invention to provide a method for producing such engine valves.